Buried word line of a dynamic random access memory and method for fabricating the same

ABSTRACT

A method for fabricating a buried word line (BWL) of a dynamic random access memory (DRAM) includes the steps of: forming a first doped region in a substrate; removing part of the first doped region to form a trench in the substrate; forming a gate structure in the trench; and forming a barrier structure between the gate structure and the first doped region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 16/406,017filed May 8, 2019, which is a division of U.S. application Ser. No.15/841,257 filed Dec. 13, 2017, all of which are hereby incorporated byreference in their entirety

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a method for fabricating semiconductor device,and more particularly to a method for fabricating buried word line (BWL)of a dynamic random access memory (DRAM) device.

2. Description of the Prior Art

As electronic products develop toward the direction of miniaturization,the design of dynamic random access memory (DRAM) units also movestoward the direction of higher integration and higher density. Since thenature of a DRAM unit with buried gate structures has the advantage ofpossessing longer carrier channel length within a semiconductorsubstrate thereby reducing capacitor leakage, it has been gradually usedto replace conventional DRAM unit with planar gate structures.

Typically, a DRAM unit with buried gate structure includes a transistordevice and a charge storage element to receive electrical signals frombit lines and word lines. Nevertheless, current DRAM units with buriedgate structures still pose numerous problems due to limited fabricationcapability. Hence, how to effectively improve the performance andreliability of current DRAM device has become an important task in thisfield.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a method forfabricating a buried word line (BWL) of a dynamic random access memory(DRAM) includes the steps of: forming a first doped region in asubstrate; removing part of the first doped region to form a trench inthe substrate; forming a gate structure in the trench; and forming abarrier structure between the gate structure and the first doped region.

According to another aspect of the present invention, a buried word line(BWL) of a dynamic random access memory (DRAM) includes: a trench in asubstrate; a doped region adjacent to two sides of the trench; a gatestructure in the trench; and a barrier structure between the gatestructure and the doped region. Preferably, the gate structure includesa conductive layer in the trench and a gate dielectric layer between theconductive layer and the substrate, and the barrier structure and thegate dielectric layer comprise different depths.

According to yet another aspect of the present invention, a buried wordline (BWL) of a dynamic random access memory (DRAM) includes: a trenchin a substrate; a first doped region adjacent to two sides of thetrench; a gate structure in the trench; and a barrier structure in thesubstrate adjacent to two sides of the trench. Preferably, the gatestructure includes: a conductive layer in the trench and a gatedielectric layer between the conductive layer and the substrate, and thebarrier structure overlaps the first doped region and the substrate.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a top view of a DRAM device according to anembodiment of the present invention.

FIGS. 2-7 illustrate cross-sectional views for fabricating a buried wordline of a DRAM device along the sectional line AA′ in FIG. 1.

FIG. 8 illustrates a structural view of a buried word line of DRAMdevice according to an embodiment of the present invention.

FIGS. 9-12 illustrate a method for fabricating a buried word line of aDRAM device according to an embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1-7, FIGS. 1-7 illustrate a method for fabricating aDRAM device according to an embodiment of the present invention, inwhich FIG. 1 illustrates a top-view diagram of a DRAM device and FIGS.2-7 illustrate cross-sectional views for fabricating a buried word lineof a DRAM device along the sectional line AA′ in FIG. 1. Preferably, thepresent embodiment pertains to fabricate a memory device, and moreparticularly a DRAM device 10, in which the DRAM device 10 includes atleast a transistor device (not shown) and at least a capacitor structure(not shown) that will be serving as a smallest constituent unit withinthe DRAM array and also used to receive electrical signals from bitlines 12 and word lines 14.

As shown in FIG. 1, the DRAM device 10 includes a substrate 16 such as asemiconductor substrate or wafer made of silicon, a shallow trenchisolation (STI) 24 formed in the substrate 16, and a plurality of activeareas (AA) 18 defined on the substrate 16. A memory region 20 and aperiphery region (not shown) are also defined on the substrate 16, inwhich multiple word lines 14 and multiple bit lines 12 are preferablyformed on the memory region 20 while other active devices (not shown)could be formed on the periphery region. For simplicity purpose, onlydevices or elements on the memory region 20 are shown in FIG. 1 whileelements on the periphery region are omitted.

In this embodiment, the active regions 18 are disposed parallel to eachother and extending along a first direction, the word lines 14 ormultiple gates 22 are disposed within the substrate 16 and passingthrough the active regions 18 and STI 24. Preferably, the gates 22 aredisposed extending along a second direction, in which the seconddirection crosses the first direction at an angle less than 90 degrees.

The bit lines 12 on the other hand are disposed on the substrate 16parallel to each other and extending along a third direction whilecrossing the active regions 18 and STI 24, in which the third directionis different from the first direction and orthogonal to the seconddirection. In other words, the first direction, second direction, andthird direction are all different from each other while the firstdirection is not orthogonal to both the second direction and the thirddirection. Preferably, contact plugs such as bit line contacts (BLC)(not shown) are formed in the active regions 18 adjacent to two sides ofthe word lines 14 to electrically connect to source/drain region (notshown) of each transistor element and storage node contacts (not shown)are formed to electrically connect to a capacitor.

The fabrication of word lines 14 (or also referred to as buried wordlines) is explained below. As shown in FIG. 2, an ion implantationprocess is conducted by implanting dopants such as n-type dopants intothe substrate 16 to form a doped region 26 or a lightly doped drain(LDD) and a trench 28 is formed in the substrate 16 thereafter. In thisembodiment, the formation of the trench 28 could be accomplished byfirst forming a patterned mask (not shown), such as a patterned resiston the surface of the substrate 16 or doped region 26, and thenconducting an etching process by using the patterned mask as mask toremove part of the doped region 26 and part of the substrate 16 underthe doped region 26 to form a trench 28.

Next, as shown in FIG. 3, an organic layer 30 is formed into the trench28 to fill the trench 28 completely and at the same time covering thesurface of the substrate 16 or doped region 26. In this embodiment, theorganic layer 30 preferably includes a resist material, but not limitedthereto. Next, an etching process is conducted to remove part of theorganic layer 30 so that the top surface of the remaining organic layer30 is substantially even with the bottom surface of the doped region 26.In this embodiment, the etching process conducted to remove part of theorganic layer 30 could include a dry etching process, in which the dryetching process could further include two stages for removing part ofthe organic layer 30 on the surface of the substrate 30 and thenadjusting the height of the remaining organic layer 30. Preferably, theetching gas used in the two stages of dry etching process could be thesame or different and the etching gas from both stages could includeoxygen, nitrogen, or combination thereof. Nevertheless, according toother embodiments of the present invention, the etching gases from bothstages of the dry etching process could also be selected from the groupconsisting of ammonia gas (NH₃), carbon monoxide (CO), carbon dioxide(CO₂), sulfur dioxide (SO₂), and carbonyl sulfide (COS).

Next, as shown in FIG. 4, a low temperature atomic layer depositionprocess is conducted to form a first gate dielectric layer 32 in thetrench 28 and on the organic layer 30. Specifically, the first gatedielectric layer 32 is deposited on sidewalls of the trench 28 above theorganic layer 30, the top surface of the doped region 26, and the topsurface of the organic layer 30.

In this embodiment, the first gate dielectric layer 32 preferablyincludes silicon oxide. Nevertheless, according to other embodiments ofthe present invention, the first gate dielectric layer 32 could alsoinclude a high-k dielectric layer depending on the demand of theproduct, in which the high-k dielectric layer is preferably selectedfrom dielectric materials having dielectric constant (k value) largerthan 4. For instance, the high-k dielectric layer may be selected fromhafnium oxide (HfO₂), hafnium silicon oxide (HfSiO₄), hafnium siliconoxynitride (HfSiON), aluminum oxide (Al₂O₃), lanthanum oxide (La₂O₃),tantalum oxide (Ta₂O₅), yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂),strontium titanate oxide (SrTiO₃), zirconium silicon oxide (ZrSiO₄),hafnium zirconium oxide (HfZrO₄), strontium bismuth tantalate(SrBi₂Ta₂O₉, SBT), lead zirconate titanate (PbZr_(x)Ti_(1-x)O₃, PZT),barium strontium titanate (Ba_(x)Sr_(1-x)TiO₃, BST) or a combinationthereof.

Next, as shown in FIG. 5, an etching process is conducted to remove thefirst gate dielectric layer 32 on the top surface of the doped region 26and the top surface of the organic layer 30 so that the remaining firstgate dielectric layer 32 only covers the sidewalls of the doped region26 or sidewalls of the trench 28 directly above the organic layer 30.Next, another etching process is conducted to remove the remainingorganic layer 30 completely. In this embodiment, the etching processconducted to remove the organic layer 30 preferably includes a wetetching process, in which the wet etching process is conducted under anenvironment between 98° C. to 120° C. by using etchant including but notlimited to for example sulfuric acid-hydrogen peroxide mixture (SPM) toremove the organic layer 30 and expose sidewalls and bottom of thetrench 28 below the doped region 26 without damaging any of the firstgate dielectric layer 32.

Next, as shown in FIG. 6, a second gate dielectric layer 34 is formed onthe first gate dielectric layer 32 and in the trench 28, in which thesecond gate dielectric layer 34 conformally covers the top surface ofthe doped region 26, the top and bottom surfaces and sidewalls of thefirst gate dielectric layer 32, and sidewalls of the trench 28 below thedoped region 26.

In this embodiment, the first gate dielectric layer 32 and the secondgate dielectric layer 34 are preferably made of same material such assilicon oxide. Nevertheless, according other embodiments of the presentinvention, the second gate dielectric layer 34 could also include ahigh-k dielectric layer having dielectric constant larger than 4depending on the demand of the product and the material of the high-kdielectric layer could be selected from hafnium oxide (HfO₂), hafniumsilicon oxide (HfSiO₄), hafnium silicon oxynitride (HfSiON), aluminumoxide (Al₂O₃), lanthanum oxide (La₂O₃), tantalum oxide (Ta₂O₅), yttriumoxide (Y₂O₃), zirconium oxide (ZrO₂), strontium titanate oxide (SrTiO₃),zirconium silicon oxide (ZrSiO₄), hafnium zirconium oxide (HfZrO₄),strontium bismuth tantalate (SrBi₂Ta₂O₉, SBT), lead zirconate titanate(PbZr_(x)Ti_(1-x)O₃, PZT), barium strontium titanate(Ba_(x)Sr_(1-x)TiO₃, BST) or a combination thereof.

In this embodiment, the thickness of the second gate dielectric layer 34is preferably slightly greater than or equal to the thickness of thefirst gate dielectric layer 32, in which the thickness of the secondgate dielectric layer 34 is between 30 Angstroms to 60 Angstroms and thethickness of the first gate dielectric layer 32 is between 20 Angstromsto 60 Angstroms.

Next, as shown in FIG. 7, a barrier layer 36 and a conductive layer 38are sequentially formed on the second gate dielectric layer 34, in whichthe barrier layer 36 is formed on the surface of the second gatedielectric layer 34 and the conducted layer 38 is formed on the barrierlayer 36 while filling the trench 28 completely. Next, an etching backprocess is conducted to remove part of the conductive layer 38, part ofthe barrier layer 36, part of the second gate dielectric layer 34, andpart of the first gate dielectric layer 32 to form a gate structure 40in the trench, a shielding or barrier structure 42 made of remainingfirst gate dielectric layer 32 between the gate structure 40 and thedoped region 26, and an air gap 44 enclosed within the conductive layer38 at the same time, in which the gate structure 40 preferably being oneof the word lines 14 shown in FIG. 1.

In this embodiment, the barrier layer 36 preferably includes TiN, butnot limited thereto. Specifically, the barrier layer 36 preferablyincludes a work function metal layer which could be a n-type workfunction metal layer or p-type work function metal layer depending onthe demand of the process or product. In this embodiment, n-type workfunction metal layer could include work function metal layer having awork function ranging between 3.9 eV and 4.3 eV such as but not limitedto for example titanium aluminide (TiAl), zirconium aluminide (ZrAl),tungsten aluminide (WAl), tantalum aluminide (TaAl), hafnium aluminide(HfAl), or titanium aluminum carbide (TiAlC), but it is not limitedthereto. P-type work function metal layer on the other hand couldinclude work function metal layer having a work function ranging between4.8 eV and 5.2 eV such as but not limited to for example titaniumnitride (TiN), tantalum nitride (TaN), or tantalum carbide (TaC), butnot limited thereto.

Viewing from a structural perspective, the top surface of the barrierstructure 42 is even with the top surfaces of the second gate dielectriclayer 34, the barrier layer 36, and the conductive layer 38 and the topsurfaces of all these four elements are slightly higher than the bottomsurface of the doped region 26. The bottom surface or bottommost pointof the barrier structure 42 is also even with or not lower than thebottom surface of the doped region 26 while the portion of second gatedielectric layer 34 directly contacting the sidewall and bottom surfaceof the barrier structure 42 forms a step-shaped profile 46. The air gap44 is enclosed within the conductive layer 38 below the doped region 26while the top surface or topmost point of the air gap 44 is lower thanthe bottom surface of the doped region 26.

Next, a hard mask 48 is formed on the gate structure 40, in which a topsurface of the hard mask 48 is even with the top surface of the dopedregion 26. In this embodiment, the hard mask 48 is preferably made ofdielectric material including but not limited to for example siliconnitride. Next, a contact plug process could be conducted to form bitline contacts adjacent to two sides of the gate structure 40electrically connecting the source/drain region and bit lines formedthereafter and storage node contacts electrically connecting the dopedregion 26 and capacitors fabricated in the later process. This completesthe fabrication of a buried word line according to an embodiment of thepresent invention.

Referring to FIG. 8, FIG. 8 illustrates a structural view of a buriedword line of DRAM device according to an embodiment of the presentinvention. As shown in FIG. 8, it would be desirable to adjust orslightly lower the depth of the first gate dielectric layer 32 byadjusting the height of the organic layer 30 when part of the organiclayer 30 is removed and first gate dielectric layer 32 is deposited onthe organic layer 30 in FIGS. 2-4. For instance, it would be desirableto slightly lower the height of the organic layer 30 within the trench28 so that the bottom surface of the first gate dielectric layer 32formed on the organic layer 30 would be slightly lower than the bottomsurface of the doped region 26. Next, processes from FIGS. 4-7 areconducted and the remaining first gate dielectric layer 32 would becomethe barrier structure 42 shown in FIG. 8, in which the top surface ofthe barrier structure 42 would be slightly higher than the bottomsurface of the doped region 26 while the bottommost surface or point ofthe barrier structure 42 would be slightly lower than the bottom surfaceof the doped region 26.

Referring to FIGS. 9-12, FIGS. 9-12 illustrate a method for fabricatinga buried word line of a DRAM device according to an embodiment of thepresent invention. As shown in FIG. 9, an ion implantation process isfirst conducted to implant dopants such as n-type dopants into thesubstrate 16 to form a doped region 52 or lightly doped drain, apatterned mask 54 is formed on the surface of the substrate 16, and partof the doped region 52 is removed by using the patterned mask 54 as maskto form at least a trench 56 in the substrate 16.

In contrast to the aforementioned embodiment of sequentially removingpart of the substrate 16 containing dopants and part of the substrate 16having no dopants, this embodiment only removes part of the doped region52 or part of the substrate 16 containing dopants while none of thesubstrate 16 having no dopants is removed. In other words, the bottom ofthe trench 56 is preferably even with the bottom surface of the dopedregion 52.

Next, as shown in FIG. 10, an ion implantation process is conducted toimplant dopants into the substrate 16 adjacent to two sides of thetrench 56 and the substrate 16 directly under the trench 56 for forminganother doped region 58. Structurally, the doped region 58 issubstantially U-shaped and the doped region 58 is disposed in thesubstrate 16 directly under and adjacent to two sides of the trench 56while overlapping part of the doped region 52 and part of undopedsubstrate 16. In this embodiment, the doped region 52 and the dopedregion 58 preferably include same conductive type such as both beingn-type doped regions and the concentration of the doped region 58 isslightly lower than the concentration of the doped region 52.

Next, as shown in FIG. 11, an etching process is conducted by using apatterned mask 54 as mask to extend the trench 56 further downward byremoving part of the doped region 58 and part of the substrate 16 toform another trench 60. It should be noted that the patterned mask 54may be slightly consumed during the etching process so that after thetrench 60 is formed, the thickness of the patterned mask 54 could becomeslightly less than the thickness of the patterned mask 54 shown in FIG.10. At this stage, the bottom of the trench 60 is preferably lower thanthe bottom surface of the doped region 52 and since the doped region 58extending from the left side of the trench 56 to the right side of thetrench 56 was separated in half as the trench 56 was extended downward,the remaining doped region 58 then becomes a barrier structure 74 madeof two disconnected doped regions adjacent to two sides of the trench60.

Next, as shown in FIG. 12, a gate dielectric layer 62, a barrier layer64, and a conductive layer 66 are sequentially formed into the trench 60to fill the trench 60 entirely, and an etching back process is conductedto remove part of the conductive layer 66 and part of the barrier layer64 to form a gate structure 68 in the trench 60 and at the same timeform an air gap 70 in the conductive layer 66. Preferably, the gatestructure 68 is one of the word lines 14 shown in FIG. 1, the topsurface of the barrier layer 64 is slightly lower than the top surfacesof the barrier structure 74 and conductive layer 66 but higher than thetop surface of the doped region 52, and the top surface of theconductive layer 66 is also higher than the top surface of the barrierstructure 74. Nevertheless, according to other embodiments of thepresent invention, the top surface of the barrier layer 64 could also beeven with the top surface of the barrier structure 74, the top surfaceof the conductive layer 66 could be even with the top surface of barrierstructure 64, or the top surfaces of the barrier layer 64, theconductive layer 66, and the barrier structure 74 are coplanar, whichare all within the scope of the present invention.

Next, a hard mask 72 is formed on the gate structure 68, in which thetop surface of the hard mask 72 is even with the top surface of thedoped region 52. In this embodiment, the hard mask 72 is preferably madeof dielectric material including but not limited to for example siliconnitride. Next, a contact plug process could be conducted to form bitline contacts adjacent to two sides of the gate structure 68electrically connecting the source/drain region and bit lines formedthereafter and storage node contacts electrically connecting the dopedregion 52 and capacitors fabricated in the later process. This completesthe fabrication of a buried word line according to an embodiment of thepresent invention.

In today's DRAM fabrication process, problem such asgate-induced-drain-leakage (GIDL) often arise when doped region such aslightly doped drain or source/drain region is too close to the gatestructure. In order to resolve this issue, the present inventionspecifically forms a barrier structure between the gate structure (orburied word line) and the doped region, in which the barrier structurecould be formed within the trench and made of same material as thesecond gate dielectric layer through the process shown in FIGS. 2-7, orcould be formed outside the trench and made by ion implantation throughthe process shown in FIGS. 9-12. By using either of these two approachesthe issue of GIDL could be reduced effectively and performance of thedevice could be improved substantially.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A method for fabricating a buried word line (BWL)of a dynamic random access memory (DRAM), comprising: forming a firstdoped region in a substrate; removing part of the first doped region toform a trench in the substrate; forming a gate structure in the trench;and forming a barrier structure between the gate structure and the firstdoped region, wherein a sidewall of the barrier structure is alignedwith a sidewall of the gate structure.
 2. The method of claim 1, furthercomprising: removing part of the first doped region and part of thesubstrate to form the trench; forming an organic layer in the trench;removing part of the organic layer; forming a first gate dielectriclayer in the trench and on the organic layer; removing the organiclayer; forming a second gate dielectric layer on the first gatedielectric layer and in the trench; and forming a conductive layer onthe second gate dielectric layer to fill the trench.
 3. The method ofclaim 2, further comprising forming the conductive layer on the secondgate dielectric layer and forming an air gap in the conductive layer atthe same time.
 4. The method of claim 2, further comprising: forming abarrier layer on the second gate dielectric layer before forming theconductive layer; removing part of the conductive layer, part of thebarrier layer, part of the second gate dielectric layer, and part of thefirst gate dielectric layer to form the barrier structure and the gatestructure; and forming a hard mask on the gate structure.
 5. The methodof claim 1, wherein a top surface of the barrier structure is higherthan a bottom surface of the first doped region and a bottom surface ofthe barrier structure is even with the bottom surface of the first dopedregion.
 6. The method of claim 1, wherein a top surface of the barrierstructure is higher than a bottom surface of the first doped region anda bottom surface of the barrier structure is lower than the bottomsurface of the first doped region.
 7. The method of claim 1, furthercomprising: conducting an ion implantation process to form a seconddoped region adjacent to two sides of the trench and in the substratedirectly under the trench; removing part of the second doped region andpart of the substrate to form the barrier structure adjacent to twosides of the trench; and forming the gate structure in the trench afterforming the barrier structure.
 8. The method of claim 7, wherein aconcentration of the second doped region is less than a concentration ofthe first doped region.
 9. The method of claim 7, wherein the firstdoped region and the second doped region comprise same conductive type.10. A method for fabricating a buried word line (BWL) of a dynamicrandom access memory (DRAM), comprising: forming a first doped region ina substrate; removing part of the first doped region to form a trench inthe substrate; forming a gate structure in the trench; and forming abarrier structure between the gate structure and the first doped region,wherein the barrier structure overlaps the first doped region and thesubstrate.